The present invention relates to a process for preparing fine fibrous whiskers of single crystals of silicon carbide (hereinafter referred to as SiC), and more specifically to a process for preparing SiC whiskers having a high homogeneity and excellent characteristics in shapes thereof.
SiC whiskers are superior in properties such as specific strength, specific elastic modulus, heat resistance, and chemical stability. Thus, they have recently been recognized as a useful reinforcing material of composites with metal, plastic, ceramic, or the like.
Preparation of SiC whiskers involves various factors such as silicon source material, kinds of carbonaceous material and third component, blending ratio, blending method, reaction atmosphere, reaction temperature, catalyst and so forth. Therefore, the quality and yield of the formed whiskers greatly depend on these factors. Accordingly, how to combine these factors has a decisive significance in obtaining SiC whiskers in a high yield. Thus there have heretofore been proposed many preparation processes with variations of the above-mentioned factors.
However, in such conventional processes for preparing SiC whiskers, investigations were made mainly on the silicon source material and the third component such as catalyst or whisker formation promotor, while no detailed investigations were made on the influence of the properties of the carbonaceous material on whiskers formed. In view of this, the inventors of the present invention have made various investigations focused, among various carbonaceous materials, on carbon black, of which the particle characteristics can be relatively easily controlled or adjusted. As a result, they have found that use of carbon black within given selective characteristic ranges as the carbonaceous material markedly improves the quality, yield, etc. of SiC whiskers formed, and proposed a process in Japanese Patent Application No. 85,906/1983 (Japanese Patent Laid-Open No. 213,700/1984).
According to the proposed process, when use as the carbonaceous material is made of carbon black having a nitrogen adsorption specific surface area of at least 40 m.sup.2 /g, a dibutyl phthalate absorption of at least 90 ml/100 g, and a tint strength smaller than or equal to a value calculated according to a formula: [0.3496.times.(BET specific surface area)-0.2143.times.(dibutyl phthalate absorption)+101.87], high-quality SiC whiskers substantially free from particulate SiC can be prepared in high yields. In order that SiC whiskers exhibit a superior performance as the reinforcing material of a composite with a metal or a plastic, however, it is necessary that the whiskers have the following characteristics in their shapes.
(a) There are few crooked or branched whiskers. The crystals are grown straight and long.
(b) The whiskers are excellent in uniformity of diameter and length, and homogeneity of aspect ratio.
In view of the above, the inventors of the present invention have made further detailed investigations on a relationship between the particle characteristics of the carbon black and the shapes of whiskers formed. As a result, they have found that, when carbon black having a nitrogen adsorption specific surface area, a dibutyl phthalate absorption, and a tint strength within respectively specified ranges is used, long whiskers can be obtained which have not only an increased aspect ratio (a length to diameter ratio of whisker) but also an improved homogeneity thereof, and which are grown straight with little deformation such as crooking or branching, and completed the present invention based on these findings.